Using a sharp knife, cut each tortilla into 4 or 5 strips, each measuring about 3⁄4 inches wide.

Pour vegetable oil to a depth of 3⁄4 inch into a heavy frying pan. Heat until a small piece of tortilla, added to the oil, floats on the top and bubbles at the edges.

Add a few tortilla strips to the hot oil and fry for a few minutes, until crisp and golden brown all over, turning them occasionally. Remove with a slotted spoon and drain on a double layer of paper towels. Cook the remaining tortilla strips in the same way.

Heat the 1 tablespoon vegetable oil in a large, heavy pan. Add the chopped onion and garlic and cook over medium heat for 2 to 3 minutes, stirring constantly with a wooden spatula, until the onion is soft and translucent. Do not let the garlic turn brown or it will give the soup a bitter taste.

Chop the tomatoes using a large, sharp knife and add them to the onion mixture in the pan. Pour in the chicken stock and stir well. Bring to a boil, then lower the heat and let simmer for about 10 minutes, until the liquid has reduced slightly.

Chop the cilantro. Add to the soup, reserving a little to use as a garnish. Season to taste.

Place a few of the crisp tortilla pieces in the bottom of four warmed soup bowls. Ladle the soup on top. Sprinkle each portion with the reserved chopped cilantro and serve.